1. Field of the Invention
The disclosure relates generally to a system and method for offshore floating structures for use in the oilfield and related industries for exploration and extraction of minerals and resources from below surface waters. More specifically, the disclosure relates to a system and method for semi-submersible offshore platforms.
2. Description of the Related Art
With the significantly increasing demand on the oil and gas supply, offshore exploration and production from reservoirs has become vital to such supply. These reservoirs usually require large drilling rigs and variable payloads which result in very large topsides in both size and weight. Large and expensive supporting offshore platforms are needed. However, the expense of such platforms can be decreased by building such a floating structure near or on shore and towing the structure to the intended offshore site.
Among the main types of offshore platforms designed for deep water, including the popular Spar, a type of platform is known as a semi-submersible platform. The structure is built near or on shore, floated to the offshore site, and the partially submerged using ballast tanks to provide stability to the structure. Semi-submersibles are typically configured with large buoyant pontoon structures below the water surface and slender columns passing through the water surface supporting a topsides deck at a significant height above the water surface. Semi-submersible platforms make large and cost effective platforms for drilling and production of offshore oil and gas. However, because the structure has a relatively large floating surface, one challenge is restricting movement caused by wave and wind action to provide a desired stability for operations.
Heave plates have been used to stabilize movement of the semi-submersible platforms. The heave plate can be a solid plate or a constructed assembly of a plurality of plates that form a box to form a relatively large horizontal surface area, but is relatively thin vertically. The heave plate is mounted to the semi-submersible platform below the water surface and below at least a portion of the wave-influenced water zones. The heave plate increases the hydrodynamic mass of the offshore platform, where hydrodynamic mass is a measure of the amount of a fluid moving with a body that accelerates in the fluid and depends on the shape of the body and the direction of its motion. The heave plate at the lower depths provides additional resistance to vertical and tilting motion that would otherwise occur near or at the water surface. Thus, designers are motivated to mount the heave plate at deeper levels. However, the depth is initially limited, because the platform is built near or on shore at shallow depths. Thus, some systems have a lowering capability to the heave plate. The heave plate can be lowered to a more desirable depth after the platform is in position at the intended offshore site. Examples of such systems are illustrated, for example, in U.S. Pat. No. 6,652,192 and U.S. Pat. No. 7,219,615 (as a continuation of U.S. Pat. No. 7,156,040), and are incorporated by reference herein. Each of these systems discloses lowering the heave plate to a depth below the platform after being located to the intended offshore site.
U.S. Pat. No. 6,652,192 discloses a heave suppressed, floating offshore drilling and production platform having vertical columns, lateral trusses connecting adjacent columns, a deep-submerged horizontal plate supported from the bottom of the columns by vertical truss legs, and a topside deck supported by the columns. The lateral trusses connect adjacent columns near their lower end to enhance the structural integrity of the platform. During the launch of the platform and towing in relatively shallow water, the truss legs are stowed in shafts within each column, and the plate is carried just below the lower ends of the columns. After the platform has been floated to the deep water drilling and production site, the truss legs are lowered from the column shafts to lower the plate to a deep draft for reducing the effect of wave forces and to provide heave and vertical motion resistance to the platform. Water in the column shafts is then removed for buoyantly lifting the platform so that the deck is at the desired elevation above the water surface.
U.S. Pat. No. 7,219,615 discloses a semi-submersible vessel having a pair of vertically spaced pontoons with varied buoyancy. The lower pontoon is retained in a close vertical proximity to the upper pontoon when the vessel is in transit. The lower pontoon is ballasted at the deployment site, dropping the pontoon to a depth of about 32 meters below the first pontoon baseline. As a result, stability and motion characteristics of the vessel are significantly improved.
While each of these systems offer solutions for a stabilized platform having a lowered heave plate, in practice the supporting structure for the heave plate to the platform may suffer from rigidity challenges. For example, U.S. Pat. No. 7,219,615 discloses extendable legs. Due to the extendable nature of the legs, no diagonal bracing between legs is shown that would be able to resist twisting and bending of the extended support structure to the heave plate, because diagonal bracing between the legs would apparently interfere with extending and retracting the legs through the guides. U.S. Pat. No. 6,652,192 illustrates extendable trusses within columns having diagonal flexible cable bracing installed between trusses after extension of the legs. Due to an interference between the truss diagonal members and the column, it is hard to design a receptacle which can enclose the truss legs and rigid diagonal bracing for effective support and load transfer. The patent does not disclose rigid bracing between trusses for the same reason, namely, the rigid bracing between the trusses would appear to interfere with extending and retracting the trusses.
Further, the designs may also suffer from strength of connections between the trusses and columns of the offshore platform. When the trusses are coupled to the columns, generally the coupling is in a cantilever fashion. A cantilever support at a corner of a column can extend outwardly to a corner of a truss assembly, so that each corner of the truss assembly can be attached to the cantilever support. For example, a four-sided truss assembly could be supported by four cantilever supports at each corner. Such an example is illustrated in FIG. 11A of U.S. Pat. No. 6,652,192. The large loads created by the truss, and the heave plate coupled to the truss, can cause design challenges in making such a cantilevered support structurally and economically efficient.
There remains a need for a different system and method for an offshore platform having an improved truss system coupled to a heave plate that can be lowered in elevation.